1. Field of the Invention
This invention relates to a laminated heated glazing comprising at least two rigid sheets, in particular of glass, and at least one interlayer carrying a heating network of fine wires of electrical resistance.
2. Background of the Related Art
It is known to use heated glazings of the type described above to equip vehicles for air, maritime, and land transport. These glazings are used in particular as windshields or side windows of aircraft cockpits and as windshields of boats and trains. These glazings comprise a network formed by fine metal wires placed in parallel between two power lead-in strips (or collectors) connected to an electric voltage source outside the glazing. These conductors placed in rectilinear directrices are most often undulated or curled along these rectilinear directrices to distribute the dissipation of the heat better, to reduce the extent of the phenomena of optical diffraction created by the presence of the network, such as to increase the length of wire for a given distance between collectors.
When the heated glazing is used as aircraft glazing, for example, it is subjected under the conditions of use to significant stresses due to the differences of pressure and temperature between the two faces of the glazing, i.e. the face oriented toward the inside of the cockpit and the face oriented toward the outside. These stresses create shearing forces causing a relative displacement of the various elements constituting the laminated structure and thereby, in particular, a displacement of the interlayer (or interlayers), causing a relative displacement of the power lead-in strips relative to the wires able to go so far as to cause the breaking of resistance wires.
The breaking of resistance wires then disturbs the temperature regulation of the glazing and causes localized overheating which also accentuates the degradation of the glazing.
To mitigate this drawback, it has been proposed to give more flexibility to the resistance wires by forming a network called "returned" as described, for example, in French patent publication 1 398 776.
In this structure, with returned network, the heating network is placed on the interlayer, and directly on the glass sheet constituting the outside element of the heated glazing while the two power lead-in strips are placed on the side of the interlayer opposite the side carrying the resistant wires, the end of these wires being bent 180.degree. to come into contact with the power lead-in strips.
The network called returned imparts to the resistance wires a flexibility which increases its resistance to the mechanical stresses resulting from the temperature differences and the pressure difference between the faces of the glazing.
This solution is generally satisfactory when the heating network is in contact with the outside glass sheet. On the other hand, this solution is not entirely satisfactory when the heating network is in contact with a rigid sheet, in particular of glass, inside the structure, and breaks of the resistance wires are then observed.
According to publication FR-A-2 310 979, a heated glazing also is known in which the heating resistance consists of a continuous electroconductive coating on which two collectors are glued. To avoid the delamination of glass plates when cold due to the expansion differences between the rigid glass sheets and the polyvinyl butyral (PVB) interlayer, a polyurethane sheet is placed between the rigid outside sheet and the PVB interlayer, and a separating frame between the PVB interlayer and the inside glass sheet.
The problem of the breaking of fine wires of electrical resistance is not dealt with in this document.
If this breaking problem of the wires is partially due to the expansion differences between the elements of the laminated glazing as is the case for the delamination of glass sheets when cold, it is mainly due to the pressure differences being exerted on the faces of the glazing, particularly in the case of an aircraft glazing. When such a glazing is at a high altitude, these differences in pressure cause a bending of the glazing.